Heretofore, photo-cationically polymerizable epoxy resin compositions have been used as one type of adhesive for use in implementing electronic components such as IC chips onto wiring substrates. Such photo-cationically polymerizable epoxy resin compositions are supplemented with photo-cationic polymerization initiators that initiate cationic polymerization by generating protons through light. Sulfonium-antimonate complexes are known as such photo-cationic polymerization initiators.
The sulfonium-antimonate complexes have, as a counter anion, SbF6− in which a fluorine atom is bonded to a metal antimony. Therefore, large amounts of fluorine ions are generated during cationic polymerization and induce migration between dissimilar metals, causing undesired corrosion in metal wiring or connection pads. Therefore, a proposition has been made to use, as cationic polymerization initiators, sulfonium-borate complexes having a tetrakis(pentafluorophenyl) borate anion [(C6F5)4B−] in which a fluorine atom is bonded to a carbon atom, instead of SbF6− (see e.g., Patent Document 1).